Mapview in R and problem with projections/crs in Pacific area

Question

I am trying to plot data in R using mapView for the grid in the Pacific that crosses the longitude 180deg. The native crs is "+proj=merc +lon_0=150 +lat_ts=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0". The example of equivalent data is:

test.coords<-as.data.frame(cbind(c(runif(15,-180,-130),runif(5,160,180)),runif(20,40,60)))
test.sp <- SpatialPointsDataFrame(coords = cbind(test.coords$V1,test.coords$V2), data = test.coords,
                                 proj4string = CRS("+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0"))
test.sp <- spTransform(test.sp, 
          "+proj=merc +lon_0=150 +lat_ts=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0")
plot(test.sp)
mapview(test.sp)

When using plot function, the points are centered, while using mapView they get split between two sides of the map (linked image). Can the view using mapView be centered on a different longitude?

Using native crs for this map does not help. I get an error.

mapview(plot, native.crs=TRUE)

Warning message: sf layer is not long-lat data

Thank you

mapView image Pacific

Answer 1

Ok, so this is a semi-optimal solution to your question for several reasons:

1. this will only work for point data
2. it may not scale well for large point collections
3. projecting to another CRS may not work anymore (though that is not really related, as the issue at hand is about visualising)

For sets of lines or polygons, we would need another approach, but I am currently not aware of a sp/sf native solution to round-tripping coordinates of an object.

library(sp)
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.2.3, PROJ 4.9.3
library(mapview)

test.coords<-as.data.frame(cbind(c(runif(15,-180,-130),runif(5,160,180)),runif(20,40,60)))
test.sp <- SpatialPointsDataFrame(coords = cbind(test.coords$V1,test.coords$V2), data = test.coords,
                                  proj4string = CRS("+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0"))

test_sf = st_as_sf(test.sp)

shift = function(x) {
    geom = st_geometry(x)
    st_geometry(x) = st_sfc(
        lapply(seq_along(geom), function(i) {
            geom[[i]][1] = ifelse(geom[[i]][1] < 0, geom[[i]][1] + 360, geom[[i]][1])
            return(geom[[i]])
        })
        , crs = st_crs(geom)
    )
    return(x)
}

mapview(shift(test_sf)) +
    mapview(test_sf, col.regions = "orange")

^{Created on 2019-12-11 by the reprex package (v0.3.0)}

I chose to use sf rather than sp for this because mapview uses sf internally and was hoping to find a general approach to this problem which could potentially be integrated.

Answer 2

Here is a workaround for polygons - it produces some warnings, but overall does the job.

# transform grid to standard latitude and longitude
# original grid in crs "+proj=merc +lon_0=150 +lat_ts=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0"
grid<-spTransform(grid,"+init=epsg:4326")
summary(coordinates(grid))

#       V1               V2       
# Min.   :-179.8   Min.   :37.17  
# 1st Qu.:-168.8   1st Qu.:51.70  
# Median :-154.3   Median :55.02  
# Mean   :-118.0   Mean   :54.65  
# 3rd Qu.:-131.4   3rd Qu.:58.31  
# Max.   : 179.8   Max.   :65.56  

out <- lapply(1:length(grid), function(i) grid[i, ])

for (i in 1:length(grid)) {

  cds <- slot(slot(slot(out[[i]], "polygons")[[1]], "Polygons")[[1]], "coords")

  cds_polygon <- Polygons(list(Polygon(cds)), ID="out.x")
  out.x <- SpatialPolygons(list(cds_polygon))
  proj4string(out.x) <- CRS("+init=epsg:4326")

  if (cds[1,1]>0) { # depending to which side one want to move polygons that are on both sides of International Date Line
    out.y <- elide(out.x, shift=c(-360,0))
  } else {
    out.y<-out.x}

  if(i==1){grid.shifted<-out.y} else {
    grid.shifted<-raster::union(grid.shifted,out.y)
  }
}

# add data in the original grid
grid.shifted <- SpatialPolygonsDataFrame(grid.shifted, grid@data, match.ID = F)

mapview(grid.shifted)